[ios] Stops HIstory Coordinator on shutdown.

There are some crashes on shutdown and they seem to be related to
History not stopping its Coordinator before some services/observers
are dealloc. This CL stops the History Coordinator before shutting the
rest of the App.

Bug: 896309
Change-Id: I7500706266157c4a841fefa21c8299c1d8627d17
Reviewed-on: https://chromium-review.googlesource.com/c/1313674Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605407}

ios/chrome/app/main_controller.mm

@@ -948,6 +948,9 @@ enum class ShowTabSwitcherSnapshotResult {
   [_browserViewWrangler shutdown];
   _browserViewWrangler = nil;
 
+  [_historyCoordinator stop];
+  _historyCoordinator = nil;
+
   [_mainCoordinator stop];
   _httpProtocolHandlerDelegate.reset();